The oligomerization of transthyretin has been studied in vitro and in
vivo. The results showed that wild-type transthyretin synthesized in v
itro in the absence of microsomes did not form dimers and tetramers, b
ut in the presence of microsomes the mature transthyretin which had be
en translocated into the microsomal lumen formed dimers and a small am
ount of tetramers which could be detected only by using a cross-linkin
g reagent. Efficiency of tetramer formation depends upon the source of
microsomes; in fact the amount of tetramer formed in liver microsomes
was much higher than that in pancreas microsomes. Transthyretin synth
esized in HepG2 cells appeared after SDS-PAGE analysis in mostly tetra
meric form, while that synthesized in transfected COS-1 cells appeared
mainly as dimers. Brefeldin A treatment and pulse-chase experiments i
n HepG2 cells showed that transthyretin tetramer was formed in the end
oplasmic reticulum. These results strongly indicate that transthyretin
tetramer is formed most efficiently in the endoplasmic reticulum lume
n of hepatocytes. Transthyretin without the signal peptide [S(-)] and
transthyretin with a mutation that prevents processing of the signal p
eptide Mse failed to form dimers and tetramers in vitro. In the transf
ected COS-l cells, however, S(-) transthyretin did form dimers while M
se transthyretin failed to oligomerize. These results show that the cl
eavage of the signal peptide and some cellular factors are required fo
r transthyretin oligomerization. (C) 1998 Academic Press.